In the context of the invention, the cornea, iris and lens correspond to the forward sections of the eye. Fundus cameras for viewing the retina do not belong to the subject matter of the invention because of the basically different optical requirements and conditions.
An ophthalmologic viewing apparatus of the type corresponding to the invention is, for example, the surgical microscope disclosed in German patent publication 4,344,770. The use of this known surgical microscope for removing cataracts takes place with an illuminating beam which impinges on the cornea. A portion of the illuminating beam passes through the cornea in the direction toward the retina and another portion of the illuminating beam is reflected by the cornea which acts as a convex mirror. In this way, a virtual image of the illuminating light source (the so-called corneal reflex) occurs. The illuminating light reaching the retina is then diffusely reflected by the retina and is scattered back to the forward sections of the eye. This light coming from the retina is therefore reddish and passes through the forward eye sections in the direction toward the viewing apparatus. In this way, this reddish light leads to a type of transillumination of the forward sections of the eye. Accordingly, an especially contrast-rich imaging of the forward sections of the eye is possible in the viewing system.
This type of transillumination is also referred to as "red reflex" illumination and requires an illuminating beam of high luminance in order to effect an adequate illumination of the forward sections of the eye. However, this makes the virtual image of the illuminating light source very bright. This image is generated by the reflection of the illuminating radiation on the convex corneal surface and disturbs the viewing of the forward eye sections because of its brightness. This virtual image is characterized as a corneal reflection and can even lead to dazzling of the viewer.
A further viewing apparatus of the type according to the invention is disclosed in U.S. Pat. No. 5,126,877. In this viewing apparatus too, the corneal reflection disturbs the viewing of the forward eye sections. In this viewing apparatus, the illuminating light impinges on the cornea parallel to the optical axis as well as inclined to this axis. The corneal reflection can therefore not only be a problem for a coaxial illumination (0.degree. illumination) but also for illuminating light which impinges on the cornea inclined to the optical axis of the viewing system. The coaxial illumination is especially advantageous for the "red reflection" illumination.